1. Field of the Invention
This invention relates to a gradation transforming method and apparatus for use in raising gradation.
2. Description of the Prior Art
In cases where an image signal is to be transformed between different color spaces, for example, between the YCC color space and the RGB color space, it often becomes necessary to carry out processing for raising the gradient of the gradation curve of the image signal. Specifically, the definition specifying with which dynamic range the signal values expressed with, for example, 8-bit, 0 to 255 levels are to be allocated to an actual scene varies for different color spaces. Therefore, when the image signal is transformed between different color spaces, it often becomes necessary to transform the gradient of the gradation curve with respect to the scene from a small gradient to a large gradient. In cases where gradation transforming process for increasing the gradient of the gradation curve is carried out with respect to the picture element values, which are expressed with integral numbers, a discontinuous portion often occurs with the picture element values, which have been obtained from the gradation transform, in a scene of gradation containing little noise, i.e. in a region in which the original picture element values change continuously. The discontinuous portion of the picture element values appears as a contour line-like artifact in a visible image reproduced from the image signal having been obtained from the gradation transform.
Discontinuity of the picture element values, which have been obtained from the gradation transform, occurs in the manner described below. For example, incases where the gradient of the gradation curve is increased by a factor of 1.2 with respect to the picture element values expressed with 8-bit, 0 to 255 levels, the original signal values of 0, 1, 2, 3, 4, 5, 6, 7, and 8 are multiplied by 1.2, and the values of 0, 1.2, 2.4, 3.6, 4.8, 6.0, 7.2, 8.4, and 9.6 are thereby obtained. In order for the thus obtained values to be approximately represented by integral numbers, the thus obtained values are rounded to the nearest integral numbers, and the values of 0, 1, 2, 4, 5, 6, 7, 8, and 10 are thereby obtained. The signal values obtained from the transform do not contain values of 3 and 9. Specifically, the values of 3, 9, and the like, disappear due to the gradation transform at an image portion, at which the picture element values should change continuously. Therefore, the discontinuous portion occurs with the picture element values.
Accordingly, in cases where the gradation transform, in which the signal values are multiplied by 1.2 and the thus obtained values are then approximately represented by integral numbers in the manner described above, is carried out on a signal, in which the picture element values change in the pattern of, for example, 2, 2, 2, 2, 3, 3, 3, 3, 4, 4, 4, 4 at a gradation part in an image, signal values of 2, 2, 2, 2, 4, 4, 4, 4, 5, 5, 5, 5, are obtained from the gradation transform. Thus an artifact occurs in the scene of gradation containing little noise in the original image. In order for the gradation containing little noise as in the original image to be obtained in the image, which is obtained from the gradation transform, it is desired that the picture element values in the aforesaid signal become continuous as in the pattern of 2, 2, 2, 3, 3, 4, 4, 4, 5, 5, 5, 5 after being transformed.